Odd noise at startup? Desktop Diagnosis anyone?

[Thump]

Info:
Happens intermittently and does not seem to be weather related (live in AZ, warm then cold and would still happen)
Seems to not care about cold or hot engine
Only hear it at prestart
Engine starts and appears to run fine at all times

Scenario:
Turn the key to prestart (watching for WTS light activity)
Hear a "pump motor-like sound from under the hood
This noise can last from 5-15 seconds and will then stop
WTS light goes off in warm weather and noise is still there
WTS light goes off after sound of relay turning off and nosie will still run
Have started before the noise goes away and nothing is apparntly wrong

Almost seems as though the rail is losing pressure and the lift pump is haveing to work to repressurize it???
The noise definitely sounds rhythmic and is not a whine like a gas fuel pump, but has a higher note and lower note when running (can't record sounds to play in a post and not gonna try to spell a sound LOL )

Any armchair diags are appreciated. The truck is getting the turbo replaced next Tues and they are supposed to look at this, too. But it wouldn't do it today when I was there, and prolly won't again for them. I need educated ideas to relay to them so they can look for something.

Thanks in advance

 

[dieselpwred]

i had the same problem it ended up being a vacume pump by the a/c dryer. the hose came off and it would run longer than normal

 

[drewRoberts]

if it is a ticking sound what you may be hearing is injector #1 and #5 opening and closeing. this occurs on all trucks and is just a start-up procedure of the ECM.